This invention relates to methods for protecting plants from frost damage.
The growing season, usually expressed as the number of frost-free days, is the determining factor of the agricultural usefulness of land. When plants are exposed to freezing conditions due to an abnormal fluctuation of the average temperatures during the growing season, temporary or permanent damage to the plants is likely to occur. Frost damage to plants occurs when ice forms from the intracellular liquid in the plant tissues. In cases where frost formation is heavy, ice rapidly spreads through the entire structure, causing the breakdown of cell walls and cell membranes. This disruption of the plant tissues can lead to bud or fruit loss or the death of the plant.
Conventional methods of frost protection involve maintaining the temperature of plant tissues above the freezing point of water. Such traditional methods for frost control are either: (1) capital intensive, as in the case of wind machines or helicopters; (2) wasteful, polluting, and prohibited by air quality control agencies in many areas, as in the case of smudge pots; or (3) have limited applicability, as in the case of over- or under-tree water application in dry, windy areas. Moreover, these methods of control cannot be practiced on a convenient work schedule in advance of frost threat, and are often ineffective and difficult to calibrate.
It is known that water can be supercooled to temperatures below 0.degree. C. without freezing. Supercooled water will freeze upon the spontaneous formation or addition of a catalyst for the water-ice phase transition. Such catalysts are known as ice nuclei. In the absence of ice nuclei, the temperature at which frost damage occurs is renduced to temperatures a few degrees below 0.degree. C. (-5.5.degree. to -9.degree. C.).
Recently, researchers have discovered that certain bacteria, particularly Erwinia herbicola and Pseudomonas syringae, can act as ice nuclei and are known as ice-nucleation active (INA) bacteria. INA bacteria are present on plant surfaces, such as leaves, buds and blossoms, at their lowest levels in the late summer and fall; they begin to increase in February, and reach their maximum levels in March-April in the Northern Hemisphere. The presence of these INA bacteria on plant surfaces causes ice nucleation in the laboratory at temperatures 5.degree. to 8.degree. C. warmer than in the absence of the INA bacteria. Consequently, the presence of INA bacteria on crops leads to increased danger of frost damage at the most critical time of the growing season. Such findings have led to attempts to inhibit the ice-nucleating activity of INA bacteria as a method for increasing the tolerance of plants to freezing temperatures.
Certain bactericides, such as antibiotics, have been known to provide an effective control of INA bacterial populations. See, Lindow, et al., J. Amer. Soc. Hort. Sci. 109(1): 48-53 (1984).
Other bactericides, such as hypochlorite (bleach), Bordeaux mixes (copper sulfate), and copper sprays, are generally used to treat other types of agricultural maladies.
To make use of the knowledge concerning the presence of INA bacteria, several new frost prevention approaches have been proposed. These methods take advantage of the fact that only a small percentage, usually less than 10%, of the total bacteria found on plant surfaces are INA bacteria. See, Lindow, "Epiphytic Ice Nucleation-Active Bacteria," Phytopathogenic Prokaryotes, Vol. 1, ch. 14, pp. 335-362 (1982). These approaches generally utilize antagonistic behavior of the remaining bacteria as they compete for a position on the plant surface.
U.S. Pat. No. 4,432,160 to Lindow describes the application of nucleation deficient microorganisms to plants to inhibit the presence and establishment of ice nucleation capable bacteria. These antagonistic microorganisms use at least one nutrient from the plant also used by the ice nucleation capable bacteria, thereby assuring the elimination of such bacteria. Likewise, U.S. Pat. Nos. 4,045,910 and 4,161,084 to Arny, et al. describe the addition of non-ice nucleating bacteria to plants prior to the onset of freezing temperatures.
Another frost prevention method involves the use of non-phytotoxic virulent bacteriophages. U.S. Pat. No. 4,375,734 to Kozloff, et al. describes the application of such bacteriophages on the plant surfaces to selectively attack the ice-nucleating bacteria and inhibit their activity.
Recent advances in the role of INA bacteria in ice formation have led to the development of antagonistic organisms genetically altered to lack enucleation capacity (INA-).